n. the trait of remaining calm and seeming not to care; a casual lack of concern

Etymologies

Probably back-formation from unconcerned.

(American Heritage® Dictionary of the English Language, Fourth Edition)

Examples

Ignore it deliberately, as a kitten stalks away with feigned unconcern from a suddenly tedious cotton-reel, in the hope that, seen afresh from the other side of the room, it will turn once more into a mouse.

That evidence of their essential "unconcern" was established by interviews and reported in Alice Kimball Smith's A Peril and a Hope: The Scientists 'Movement in America, 1945-1947 (MIT Press, 1970, pp. 60-61.)

Donning an unadorned and boyish wardrobe that itself did not "weigh heavily," she fashioned renunciation into a contrary look of simplicity, ineffability, transcendent unconcern her competitor Paul Poiret called it "poverty deluxe".